1. Field of The Invention
The present invention relates generally to a system and method for controlling an ignition timing for an internal combustion engine to suppress engine knocking using a pseudo random signal of minute period.
2. Description of The Background Art
Japanese Patent Application First Publication (Unexamined) No. Showa 61-182465 published on Aug. 15, 1986, exemplifies an ignition timing controlling system in which ignition timing is controlled so as to advance toward an angular position in a vicinity to a knocking limit over which engine knocking occurs and below which no engine failure occurs in order to reduce fuel consumption of the engine.
As appreciated from FIG. 9, as the ignition timing is gradually advanced, an abrupt fuel combustion of the engine is advanced. Accordingly, a level of a signal representing a knocking level becomes increased. A threshold level is provided at a, so-called, knocking limit (an amplitude position over which the knocking level must not exceed to prevent engine deterioration). If the threshold level is compared with an actual knocking level and, then, the actual knocking level exceeds the threshold level, a signal representing a high level (logic high level) is output. A control unit receives the signal, i.e., a determination signal that the signal indicates the high level and controls the ignition timing so as to retard stepwise over a constant width of crank angle when a duration of time during which the high level signal is continued exceeds a predetermined period of time. Then, the control unit progressively advances the ignition timing from the retarded timing angle. In this way, the engine is controlled to an ignition timing in the vicinity to the knocking limit.
However, in the previously proposed ignition timing controlling system, the ignition timing is retarded only when the actual knocking level has exceeded the threshold level for the predetermined period of time. During the time in which the actual knocking level exceeds the threshold level, it follows that the occurrence of knocking cannot be avoided. In other words, as long as the knocking level is exceeded to some degree, the ignition timing is not optimally controlled toward the vicinity to the knocking limit. Consequently, control accuracy of the ignition timing is not always optimum.